The present invention relates to an apparatus for conditioning and conveying molten thermoplastic material produced in a melting furnace. The apparatus receives the thermoplastic material from the furnace and discharges the same at some desired temperature and consistency for forming a product. The invention more particularly pertains to a glass delivery system and flow control device wherein the glass is conveyed through a conduit and is simultaneously conditioned to a desired relatively uniform temperature and viscosity as it passes therethrough. The conduit may be immersed in or surrounded by stagnant glass. Heaters, adapted to operate in molten glass located about the conduit, provide uniform heat input into the glass within the conduit and prevent localized hot spots. The conduit may be coupled to a forehearth for further conditioning, and to downstream glass forming equipment via a glass corrosion resistant flow control device.
Vertically oriented electric glass melting furnaces have been known in the prior art for some time, but it has been only in the last several years that such furnaces have been brought to large scale commercial application. In more recently developed furnaces, such as the type illustrated in U.S. Pat. Nos. 2,993,079, 3,524,206, 3,583,861, 3,725,558, 3,742,111, 3,942,968, 4,029,887 and 4,143,232, glass forming batch materials are fed to the upper end of a vertical chamber and refined molten glass is withdrawn from the bottom of the chamber. High quality glass is thus produced in a single vertical chamber, with melting occurring in an upper portion thereof and preferably some refining occurring at the bottom portion.
The molten glass withdrawn from the electric furnace is usually received within a laterally extending connected channel situated at one side of the furnace bottom and thereafter directed through a vertical passageway or riser to a mixing chamber and/or a forehearth. In some glass melting systems there is a differential between the glass level in the furnace and the glass level in the forehearth resulting in a measurable hydrostatic head (sometimes hereinafter referred to as pressure or head). Normally, a flow control device or needle located between the furnace and forehearth controls the flow or volume of glass into the forehearth so as to regulate the head.
In a conventional flow control system, glass under hydrostatic pressure may ultimately work its way through the refractory joints and bypass the flow control device, with flow control being thereby rendered inoperative. A premature and expensive furnace repair is thus required.
Hydrostatic head may be reduced by changing the difference between glass level in the furnace and forehearth. Alternatively, the head may be dissipated by lengthening the connected channel of the delivery system. Each respective alternative lowers the tendency of the glass to bypass the needle. However, neither alternative is particularly desirable because hydrostatic head is a useful driving force for the glass, and a long connected channel is expensive to build and operate.
The present invention of an improved glass delivery system and flow control device has permitted the continued use of a relatively high hydrostatic head as a driving force for the glass. At the same time, an increase in the length of the delivery system to reduce head at the flow control device is not required.
It is well known in the glass melting and forming art that forehearths require substantial amounts of heat energy in order to condition the glass received from the furnace. Thus, the forehearth is a net consumer of energy and the anomalous condition exists wherein a large amount of heat energy is required to "cool" the glass to the proper forming temperatures. The present invention allows for the use of a smaller and lower energy consuming forehearth.
It is also known that molybdenum metal (moly), a preferred glass contact material used herein, has significantly higher wear resistance to moving molten glass than conventional refractory materials. However, it is also well known that moly tends to oxidize at temperatures in excess of 550.degree.-600.degree. C., and thus, the moly must be protected from deleterious atmosphere (oxygen) when it is used at or above these elevated temperatures. The present invention uses moly extensively and provides for its protection.
The present invention performs the functions of transportation, cooling, homogenizing and flow control of molten glass, wherein the useful life of the delivery system is significantly increased and glass-refractory corrosion products, producing glass inhomogeneities and inclusions, are virtually eliminated. Further, the system greatly reduces the net energy required to condition the glass while maintaining a relatively stable hydrostatic driving force for the glass through the system.
In a series of related U.S. patent applications, Ser. Nos. 244,024, 244,022, 244,001, 243,811 filed Mar. 16, 1981 and assigned to the assignee herein, various arrangements of glass melting and transport systems are disclosed in detail. It should be understood that although significant differences exist with such disclosures and the present invention, to the extent necessary, the teachings of said applications are to be considered incorporated herein by reference.